Notwithstanding the number of airlines you have worked for, I would follow their SOP's while operating for them.
The call is "decel" meaning deceleration. Perhaps "Decal" is a typo.
From the FCTM, the green DECEL light comes on at 80% of the selected rate. So, you may be decelerating long before the green DECEL indication. It is not therefore an indication of autobrake operation.
From my company SOP's, "The deceleration is felt by the flight crew and confirmed by the speed trend on the PFD. The deceleration may also be confirmed by the DECEL light (if autobrake is on).
So to answer your question, yes, it is based on crew sensing the deceleration. I wouldn't say that idle power guarantees a deceleration ( however I accept it would be rare not to) but you may encounter aquaplaning on occasion and need to announce the state of the aircraft with respect to braking and decelerating. It's all good situational awareness and crew cooperation/crm.
Autobrake does not relieve the pilot of the responsibility of achieving a safe stop wthin the available runway length.
Also, proper briefings should cover the use of autobrake and hopefully mitigate any misunderstandings.
Hope this helps.
Last edited by Rubber Dog; 29th Sep 2014 at 07:12.